1. Field of the Invention
The invention relates to methanation processes. More particularly, it relates to such processes having enhanced efficiency and improved operating control.
2. Description of the Prior Art
The catalytic hydrogenation of carbon monoxide is one of the most well known and established hydrogenation reactions. This reaction, which is: EQU CO+3H.sub.2 .fwdarw.CH.sub.4 +H.sub.2 O, (1)
utilizes a synthesis gas, as from the gasification of coal with oxygen and steam, that is treated to provide a desired H.sub.2 /CO ratio and to remove excess CO.sub.2 and deleterious impurities such as sulfur compounds. As the H.sub.2 /CO ratio of the raw synthesis gas is substantially below the necessary minimum ratio of 3/1, at least a portion of the carbon monoxide in the synthesis gas is generally first reacted with steam, over iron or another suitable catalyst, in the well-known "water gas shift" reaction, as follows: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2. (2)
Excess CO.sub.2 in the gas stream is removed by conventional means, such as by treatment with alkaline absorbents. Sulfur impurities are also removed to substantially under 5 ppm, e.g. to less than about 1 ppm, preferably to less than 0.2 ppm, to protect the methanation catalyst from poisoning by such sulfur impurities.
In carrying out reaction (2), the aim is normally to shift, i.e. to make hydrogen, as far as is economically possible, with the equilibrium being determined by the reaction temperature and other operating conditions employed. Excess steam is thus employed to control the reaction temperature. The desired H.sub.2 /CO ratio is obtained, to achieve maximum utilization of the available CO and hydrogen, either by very careful choice and control of the processing conditions, difficult to achieve in continuous processing operations, or by the treatment of a portion of the raw synthesis gas to produce a H.sub.2 /CO ratio substantially in excess of 3/1 and blending the thus-treated gas with the untreated portion to produce the desired H.sub.2 /CO ratio.
The latter approach is disclosed by the Muller patent, U.S. Pat. No. 3,854,895, which discloses dividing the gas produced by the pressure gasification of coals, and having a low H.sub.2 /CO ratio, into two streams for separate treatment. One stream is subjected to water gas shift conversion, i.e. reaction (2), and subsequent carbon dioxide removal, and the other stream is not so converted. The ratio of the converted stream to the unconverted stream is between 5:1 and 1:1. The two streams are separately purified to remove sulfur and other catalyst contaminants. The residual CO.sub.2 concentration of the converted stream is established at less than 3 vol.%. The CO.sub.2 concentration of the converted stream is varied only slightly as a result of a small amount of absorption of CO.sub.2 in the purification operation. In the water gas shift conversion preceding purification, the converted stream is adjusted to a residual CO concentration of about 4% by volume. The converted stream passing to the methanation stage is thus rich in hydrogen and poor in both CO and CO.sub.2. The converted stream is passed to a first methanation reactor 30, in first methanation stage F, and thereafter to reactors 31-36 thereof in which said stream is progressively mixed with the unconverted, low H.sub.2 /CO stream. Each of the reactors 31-36 is followed by condensing systems 37-42, which may include a feed water preheater and a waste heat boiler. Muller states that the ratio of the two partial streams is not varied and that the unconverted stream is divided between the reactors of methanation system F subsequent to the initial reactor 30.
Muller notes that in the temperature range of the highly exothermic methanation reaction, i.e. reaction (1), the formation of free carbon by the Boudouard reaction. that is: EQU 2CO.fwdarw.CO.sub.2 +C, (3)
is possible and is promoted by an increasing concentration of CO in the reaction mixture. Muller also notes that formation of carbon can be suppressed by operating with a high excess of hydrogen, which assumably depresses the carbon monoxide content of the reaction mixture. If such a high excess of hydrogen is sustained throughout the methanation operation until essentially all of the carbon monoxide is fully reacted, the methane product will necessarily contain an undesirable excess of hydrogen. By operating the adiabatic methanation first reaction zone of methanation stage F with said converted stream having a large excess of hydrogen, Muller proposes to suppress carbon deposition while being able to reduce the hydrogen content of said stream by mixture with portions of the unconverted stream prior to passage to subsequent methanation reaction zones.
The first methanation stage F of Muller is an adiabatic cold feed quench reactor system in which the elevated temperatures produced by the highly exothermic methanation reaction are systematically reduced by the regulated admixture of cold feed gas. Thus, the product gas from reactor 30 is cooled, thereby generating high pressure steam, mixed with a portion of the unconverted stream, and passed to reactor 31. The remaining portion of the unconverted gas is divided and mixed with the effluent gas from all but the last reactor 36 of methanation stage F. Heat generated in each exothermic methanation zone is used to produce high pressure steam. Muller discloses three systems useful for reaction zones 31-36, namely adiabatic shaft reactors, isothermal tubular reactors, and a staged water gas shift reactor and tubular reactor. The methane produced in stage F is thereafter compressed, subjected to a final methanation trim reactor, and delivered as the product synthetic natural gas stream.
Muller suppresses carbon deposition via the Boudouard reaction on the methanation catalyst by maintaining an excess of hydrogen in the feed gas. While the kinetics of the Boudouard reaction, i.e. reaction (3), are such that negligible carbon may be formed in any event at temperatures below about 600.degree. C. (i.e. below about 1112.degree. F.), the reversible decomposition of methane into hydrogen and free carbon, that is EQU CH.sub.4 .fwdarw.C+2H.sub.2 ( 4)
has been found to pose a problem at the high temperature condition of the methanation reactor effluent although not thermodynamically favored at the inlet conditions. For this reason, it is important to maintain a high relative concentration of hydrogen to methane in the high temperature adiaabatic methanation reactor effluent. This can be accomplished by maintaining a large H.sub.2 /CO ratio in the adiabatic reactor feed gas thereby assuring that sufficient hydrogen remains after methanation to prevent carbon formation by decomposition of methane product. Muller approaches this problem by attempting to avoid carbon deposition, via the Boudouard reaction, by likewise maintaining a large excess of hydrogen in the feed gas.
Another concern in the operation of a high temperature adiabatic methanation reactor is the potential for sintering of the methanation catalyst at high temperatures. The methanation reaction temperature is directly related to carbon monoxide conversion. Thus, a temperature rise of about 100.degree. F. occurs for every 1% of carbon monoxide converted. If too much carbon monoxide is present in the first stage methanation feed, its complete conversion may generate sufficient heat to partially fuse the methanation catalyst, thereby limiting its catalytic activity. The concentration of carbon monoxide in the feed gas, therefore, must be carefully regulated. Muller, as noted above, discloses a CO content of 4%, which is within the requisite feed gas concentrations, although also relying upon the use of a cold feed quench reactor system to avoid the problem of sintering.
To regulate the H.sub.2 /CO ratio and the carbon monoxide content of the feed gas to the adiabatic reaction zone, Muller attempts to control the output of the water gas shift converter. Shift converters are typically operated with excess steam to produce an effluent gas with a low carbon monoxide content, e.g. below about 2% CO. The excess steam not only forces the equilibrium shift reaction to hydrogen and carbon dioxide products but also acts as a temperature control for the reactor. The steam serves, in effect, as an intrinsic heat sink for the exothermic shift reaction. By minimizing high temperature operation and excessive temperature excursions, a higher level of catalytic activity can be maintained in the shift reactor for longer periods of time. To suitably increase the output concentration of carbon monoxide from the water gas shift reactor to the levels required for the adiabatic methanation operation, however, Muller must employ near or below stoichiometric quantities of steam in the shift reactor. Under such conditions, a higher average operating temperature in the shift converter will result and will increase the likelihood of catalyst deactivation by sintering. Any change in catalyst activity, as by such sintering, coupled with changing water gas shift equilibrium conditions will produce fluctuations in the output of the shift converter. A fluctuating converter output can lead to catalyst sintering and carbon deposition in the subsequent methanation reaction zone when the resulting carbon monoxide concentration is high.
If, on the other hand, product carbon monoxide concentration is high and the temperature is low, metal carbonyl formation becomes a problem, particularly troublesome when an adiabatic cold feed quench reactor is employed. To overcome the problem caused by overheating, extensive recycle streams are commonly employed as diluent to adsorb some of the exothermic heat evolved. Additional measures for avoiding too high temperatures in the reactor include cooling of the catalyst bed or of the reaction gases. Local heating is difficult to avoid when internal cooling of the reactor is employed, however, and the building of internal exchange surfaces tends to be expensive. Gas recycle methods, on the other hand, require high recycle ratios, and, as a consequence, large pressure drops through the catalyst beds occur. As a result, the requirements for compressor power increase proportionately, thereby increasing compression construction costs.
There exists in the methanation art, therefore, a genuine need to avoid carbon deposition and catalyst sintering, while avoiding the necessity for excessive product gas recycle or other relatively expensive means for temperature control. In addition, the methanation operation, to be of enhanced efficiency in continuous production operations, must not be unduly sensitive to fluctuations in the water gas shift converter output as a result of declining shift catalyst activity and/or fluctuations in the CO content of the high pressure gasification effluent or other synthesis gas feed stream to the shift converter.
The adabatic cold feed quench reactor system referred to above has a low steam generation potential, which is disadvantageous with respect to the overall efficiency of an overall gasification-methanation system. Methane can typically be formed either directly in the coal gasification system or in a separate methanation system subsequent to gasification. Thermodynamically, it is more efficient to form methane in the gasifier than in a methanation system because of the higher operating temperatures possible in a coal gasification system and, accordingly, of the greater available energy. To insure significant methane production, however, the gasifier must operate at a very high pressure at which the formation of tars and heavy oils is promoted. Likewise, the problems associated with the feeding of coal solids to a high pressure environment, as well as those relating to the handling of potentially carcinogenic tars and oils, are formidable.
Lower pressure gasifiers are easier to operate and are generally advantageous, therefore, but are not as thermodynamically efficient as very high pressure gasifiers since the methanation operation must be performed in a separate methanation system subsequent to the gasification of coal or other solid carbonaceous material. A need exists in the art, therefore for improvements in the overall efficiency of lower pressure gasifier-methanator systems so as to make such systems competitive, energywise, with very high pressure gasifier systems.
It is an object of the invention to provide an improved methanation process.
It is another object of the invention to provide a process for enhancing the overall efficiency of a coal gasification-synthesis gas methanation system.
It is another object of the invention to provide a process for obviating carbon deposition and catalyst sintering in continuous methanation processes without excessive recycle of effluent gas.
It is a further object of the invention to provide an enhanced methanation process not subject to carbon deposition and catalyst sintering resulting from declining water gas shift catalyst activity and/or fluctuations in the feed gas rate in continuous production operations.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.